One speed limit on your ability to upgrade your systems
[...] it's not "pain" - it just doesn't happen. At 2 weeks of planning + testing = 26 systems per year
This was eye-opening in a 'I hadn't thought about it that way before now' way. Like many insights, it's blindingly obvious in retrospect; of course how fast you can actually do an upgrade/update cycle determines how many of them you can do in a year (given various assumptions about manpower, parallelism, testing, and so on). And of course this limit applies across all of your systems. It's not just that you can only upgrade a given system so many times a year; it's that you get only so many upgrades in a year, period, across all of your systems.
(What the limit is depends very much on what systems you're trying to upgrade, since the planning, setup, and testing process will take different amounts of time for different systems.)
To upgrade systems more frequently, you have two options. First, you can reduce the time an upgrade cycle takes by speeding up or doing less planning, building, testing, and/or the actual deployment. Second, you can reduce the number of upgrades you need to do creating more uniform systems, so you amortize the time a cycle takes across more systems. If you have six special snowflakes running completely different OSes and upgrading each OS takes a month, you get twelve snowflake upgrades in a year (assuming you do nothing else). But if all six run the same OS in the same setup, you now get to upgrade all six of them more or less once a month (let's optimistically assume that deployment is a snap).
I see this as an interesting driver of uniformity (and at all levels, not just at the system level). Depending on how much pre-production testing you need and use, it's also an obvious driver of faster, better, and often more automated tests.
(Looking back I can certainly see cases where this 'we can only work so fast' stuff has been a limiting factor in our own work.)